1. Field of the Invention
The present invention relates to a substrate suitably used for a mask or a mask blank for EUV lithography.
2. Discussion of Background
In the semiconductor industry, a photolithography method using visible light or ultraviolet light has been employed as a technique for writing, on a Si substrate or the like, a fine pattern, which is required for forming an integrated circuit comprising such a fine pattern. However, the conventional photolithography method has been close to the resolution limit, while microsizing of semiconductor devices has been accelerated. In the case of the photolithography method, it is said that the resolution limit of a pattern is about ½ of an exposure wavelength, and that even if an immersion method is employed, the resolution limit is about ¼ of an exposure wavelength. Even if an immersion method using an ArF laser (193 nm) is employed, it is estimated that the resolution limit is about 45 nm. From this point of view, EUV lithography, which is an exposure technique using EUV light having a shorter wavelength than ArF lasers, is considered to be promising as an exposure technique for 45 nm or below (refer to Non-Patent Document 1). In this specification, “EUV light” means a ray having a wavelength in a soft X-ray region or a vacuum ultraviolet ray region, specifically a ray having a wavelength of from about 10 to 20 nm, in particular, of about 13.5 nm±0.3 nm.
EUV light is apt to be absorbed by any substances and the refractive indices of substances are close to 1 at this wavelength, whereby it is impossible to use a dioptric system like a conventional photolithography employing visible light or ultraviolet light. For this reason, for EUV light lithography, a catoptric system, i.e. a combination of a reflective photomask (hereinafter referred to as “EUV mask”) and a mirror, is employed.
A mask blank is a stacked member for fabrication of a photomask, which has not been patterned yet. In the case of an EUV mask blank, it has a structure wherein a substrate made of glass or the like has a reflective layer to reflect EUV light and a absorber layer to absorb EUV light, formed thereon in this order. As the reflective layer, a multilayer reflective film is usually used wherein high refractive index layers and low refractive index layers are alternately stacked to increase a light reflectance when irradiating the layer surface with EUV light. For the absorber layer, it is common to employ a material having a high absorption coefficient for EUV light, specifically e.g. a material containing Cr or Ta as the main component. As the substrate, a material having a low thermal expansion coefficient and thereby showing no deformation even under EUV light irradiation, is required, and e.g. a glass having a low thermal expansion coefficient is considered for the substrate.
Patent Documents 1 to 3 describe examples of preferred properties of the substrate for EUV masks. Further, Patent Document 4 describes a preferred level of striae of a substrate for EUV masks, and the document specifically recites that “According to certain embodiments of the present invention, applicants have demonstrated that striae in silica-titania ultra low expansion glass boules can be reduced by modification of several manufacturing parameters during flame hydrolysis. Applicants have been able to manufacture boules and extreme ultraviolet elements having rms striae values less than about 0.05 MPa, preferably less than about 0.03 MPa, and more preferably less than about 0.02 MPa. Peak to valley striae values were reduced to less than 0.2 MPa and preferably less than 0.15 MPa.”.
Further, as the method for measuring a striae level, Patent Document 4 describes that “Thus, the polariscope measures retardance through a sample as a function of position. The spatial resolution of a polariscope is much smaller than the size of the striae in titania-silica glass and therefore allows for measurement through striae layers. The retardance observed in the polariscope indicates stresses between striae layers, which are most likely due to thermal expansion mismatch between the layers. FIG. 3 shows a comparison of striae measurements made on a sample. The lower line in FIG. 3 represents striae measurements made by a polariscope, and the upper line represents measurements made by a microprobe. The polariscope used is available from Cambridge Research Instrumentation, Model LC, which was used with a Nikon microscope. As indicated by FIG. 3, there is good correlation between the two techniques, which shows that the polariscope can be used to measure striae in titania-silica glass and optical elements such as extreme ultraviolet lithographic elements.”.
However, this Patent Document 4 describes a process for producing a glass body (it is called as boule) before being cut into a substrate, but this does not clearly describe as to a production process for producing a substrate achieving the above striae level. Further, in the process described in Patent Document 4, it is unclear how the stress is measured. Further, in the process of Patent Document 4, although the detail is unclear, it seems that the stress of a glass called as a boule which is to be cut into a substrate, is measured, but it is not possible to determine the measured stress as the stress of the substrate itself. Further, Patent Document 5 and 6 describe the film constructions of EUV mask blanks.
Patent Document 1: U.S. Pat. No. 6,465,272
Patent Document 2: U.S. Pat. No. 6,576,380
Patent Document 3: U.S. Pat. No. 6,931,097
Patent Document 4: U.S. Pat. No. 7,053,017
Patent Document 5: JP-A-2004-6798 (U.S. Pat. No. 7,390,596)
Patent Document 6: U.S. Pat. No. 7,390,596
Patent Document 7: JP-B-63-24937
Patent Document 8: JP-A-2007-213020 (US-A-2008/311487)
Patent Document 9: US-A-2008/311487
Patent Document 10: WO00/75727
Patent Document 11: U.S. Pat. No. 6,352,803
Non-Patent Document
Non-Patent Document 1: Extreme ultraviolet lithography C. W. Gwyn et al J. Vac. Sci. Tech. B 16(6) No. December 1998